In draping a parison into a double-walled box or container, a lower region of the parison is clamped closed and held in place while the parison is draped over or into a mold part, with the parison sometimes being ballooned in the draping process. Preferred ways of doing this are shown in copending and commonly assigned patent applications Ser. No. 868,294, filed May 28, 1986, entitled DRAPED PARISON BLOW MOLDING now U.S. Pat. No. 4,828,786, issued May 9, 1989; Ser. No. 895,329, filed Aug. 11, 1986, entitled CORE PINCH-OFF FOR BLOW MOLDING PARISON; and Ser. No. 945,020, filed Dec. 22, 1986, entitled REVERSE PARISON DRAPING FOR BLOW MOLDING, now U.S. Pat. No. 4,770,839, issued Sept. 13, 1988.
Draping a parison around a rectangular-shaped mold part poses special problems. The preferred shape for the original parison is cylindrical, and forming a cylindrical parison to spread outward and drape over corners of a rectangular mold part requires either making the initial parison relatively large in diameter or carefully stretching a smaller parison outward into a rectangular form. In turn, enlarging or stretching the parison creates the problem of a longer linear length of clamp being required to flatten the bottom of the parison along a clamp line. It is even possible, and undesirable, for the clamp line to be longer than the long axis of the rectangular shape over which the parison is being draped. It is also possible to collapse an open bottom of a parison by blowing air downward through it, and out the open bottom, so that the parison fits within a smaller clamp opening; but I have found that a cylindrical parison collapses irregularly in response to an air jet so that the parison collapse is not repeatable and cannot produce uniform and predictable results.